SPC AC 251604

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1104 AM CDT Fri Aug 25 2017

   Valid 251630Z - 261200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ALONG THE MIDDLE
   AND UPPER TX COAST...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
   THE NORTHERN PLAINS...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE
   COASTAL PLAIN OF TX...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
   OF THE NORTHERN PLAINS...

   ...SUMMARY...
   Tornadoes and locally damaging winds are possible today and tonight
   with stronger cells mainly north through northeast of the center of
   Hurricane Harvey.  Otherwise, a cluster of thunderstorms should
   develop across parts of South Dakota and northern Nebraska, offering
   locally damaging winds and/or hail during the afternoon and evening.

   ...Harvey...

   Hurricane Harvey, currently located roughly 70 mi off the lower TX
   coast, continues its slow northwest movement toward the middle TX
   coast. Stronger outer bands are beginning to affect near-coastal
   regions and supercell structures should become more common later
   this afternoon/evening, especially along the upper TX coast where
   buoyancy is expected to be greater. Tornado threat will increase as
   these stronger structures move inland. Ref MCD #1564 for more
   details regarding the tornado threat and NHC for more information on
   Hurricane Harvey.

   ...Northern/Central Plains...

   Long-lived MCV, from overnight convection, has shifted across
   southeast MT/northeast WY to near Y22 along the ND/SD border.
   Considerable amount of clouds currently extend from the south side
   of the Black Hills into northeast SD north of the wind shift but
   strong surface heating is expected to persist across the warm
   sector. Latest short-range guidance suggests surface temperatures
   will warm to near 90F across north-central NE into south-central SD
   which should be adequate for CIN reduction necessary for
   surface-based thunderstorm development near the boundary (likely
   after 22z). Latest HRRR guidance supports this scenario with
   southwest-northeast oriented boundary becoming convectively active
   by late afternoon. Additionally, low-level warm advection should
   encourage thunderstorm activity downstream into southwest MN. Only
   minor changes have been made to the 13z outlook, namely to extend 5%
   severe probs toward the south side of the Black Hills where upslope
   flow may contribute to isolated strong thunderstorms later this
   afternoon that would shift southeast toward the NE border.

   ..Darrow/Elliott.. 08/25/2017

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